What is rewind on DJ hero 2 and how do you use it?

Rewind does exactly what the word means, nothing extra. Its used to rewind to play a part of the song over to score more pts. First you have to keep a streak going until a green rewind symbol lights up on the bottom right hand side of you screen, when that happens then if you have the buttons on the left side you spin the turn table quickly(counter clockwise), if the buttons on the right then you spin quickly(clockwise)